Is it permissible to supplicate against an abusive paternal uncle who severs ties of kinship and slanders reputations, and is it permissible to confront him verbally to stop his harm?

The best approach is to forgive and pardon the uncle, based on Allah's saying: "So whoever pardons and makes reconciliation - his reward is due from Allah." (Quran 42:40), and His saying: "But whoever is patient and forgives, indeed, that is from the matters [requiring] determination." (Quran 42:43). And based on the hadith: "If you are as you say, then it is as if you are feeding them hot ashes." The uncle might also reconsider his actions through good treatment, as Allah says: "Repel [evil] with that which is better." (Quran 41:34). Nevertheless, retaliation and responding to a bad deed with its like, without exceeding, is permissible, as Allah says: "The recompense of an evil deed is an evil deed like it." (Quran 42:40), and His saying: "except for him who has been wronged." (Quran 4:148). Supplicating against him is also permissible, as the Prophet (peace be upon him) did. However, forgiveness is the highest of stations.